This invention is generally concerned with a heat resistive barrier for use in a space vehicle, and is specifically concerned with a wall assembly that acts as a combination heat barrier and fuselage wall structure for use in a variety of aerospace vehicles, including reusable reentry vehicles, launch vehicle components, rocket boosters, and external tanks.
Heat barriers for protecting reusable space vehicles from the heat of reentering the atmosphere are known in the prior art. Such heat barriers may take the form of a ceramic tile used in the space shuttle of the United States space program. While the U.S. Space Shuttle has demonstrated that a ceramic tile type heat barrier can successfully protect the interior of the space vehicle from the heat of reentry, the use of such tiles is a major contributing factor to the high cost and the risks of space travel in at least five respects. First, the tiles themselves are relatively heavy and bulky compared to the other space-age materials which form the balance of the vehicle, which increases the overall weight of the vehicle and hence the amount of propellant needed to accelerate the vehicle to critical velocity. Secondly, the tiles are expensive to fabricate and difficult to adhere onto the fuselage of the vehicle. Thirdly, under certain atmospheric conditions, liquid water can collect beneath the tiles during reentry and tear them from the fuselage when the heat radiated by the tiles boils the water into steam. These last tiles must, of course, be replaced before the shuttle flies again, thereby necessitating the fabrication and adherence of still more of the expensive tiles. Fourthly, the ceramic tiles do not significantly assist the fuselage of the vehicle in achieving either structural strength, or the insulation of cryogenic fuel components from the harsh sunlight of space. Hence the fuselage must be provided with other structural components to give it the necessary tensile and buckling strengths, and must further be provided with cryogenic insulation structures that are entirely separate from the tiles. Finally, while the number of tiles that have been lost during past space shuttle missions has not been enough to jeopardize the thermal integrity of the vehicle, it is significant enough to call the long-term reliability of such a design into question.
Heat resistive wall structures for near-space vehicles are also known in the prior art, such as the fuselage wall of the X-15 rocket plane that was developed by North American Aviation, Inc. for NASA over twenty-five years ago. The X-15 wall structure relied primarily upon a combination of radiation cooling and a heat sink structure to resist heat. Such a wall structure has the advantage of doubling as both a structural wall and a thermal protection system for the vehicle. It is also easier to fabricate and to assemble than the tiles used in the space shuttle. But, while the use of Inconel.RTM.as the outer skin of the fuselage did provide a relatively strong and reliable structure that resisted heat better than titanium or aluminum, this wall is relatively heavy, and not capable of safely withstanding heat fluxes of the magnitude associated with atmospheric reentry. Moreover, this prior art wall does not, by itself, provide any significant cryogenic insulation for the liquid oxygen and fuel components used in space vehicles. Nor could such a wall be easily modified to provide lightweight, cryogenic insulation, since the maximum temperature sustained by the Inconel.RTM. skin would destroy internally added insulation.
Other types of heat resistive insulation structures have been studied and laboratory tested by NASA such as stand-off radiation heat shields. These heat shields typically include an inner structural wall formed of aluminum, several inches of high-temperature insulation such as Refrasil.RTM., and a corrugated outer structural wall. While these structures offer a structurally strong heat barrier that can provide some cryogenic insulation, they are heavier than ceramic tiles, and bulkier. Clearly, there is a need for a heat resistive wall capable of resisting the heat fluxes associated with reentry but which retains all the advantages associated with heat-resistive fuselage walls of rocket plane type vehicles. Such a wall structure should have a high tensile and buckling strength to minimize the need for reinforcing components that would add weight to the vehicle. It would be desirable if the wall had an inner component of high thermal conductivity and heat capacity so that any localized areas of high heat flux are quickly absorbed and distributed over wider portions of the entire structure. Ideally, such a wall structure should have the ability of effectively insulating cryogenic fuel and oxidizer components during the prelaunch period and from the harsh sunlight of space so that the necessity of separate cryogenic insulation structures is minimized. Finally, the heat resistive wall should be easy to assemble, highly mechanically reliable, extremely lightweight and easily repairable in the event of localized damage.